Recently, PhD candidate ZHANG Xiaojuan and Prof. DENG Linhua from Yunnan Observatories, Chinese Academy of Sciences (CAS) investigated the temporal variation of the hemispheric asymmetry of high-latitude and low-latitude coronal mass ejections (CMEs) during the time interval from 1996 to 2020. This work was published in Monthly Notices of the Royal Astronomical Society.

The hemispheric asymmetry phenomenon of solar magnetic field structures is a manifestation of the evolution of the solar activity cycle, discovered as early as the middle of the 20th century. From a statistical perspective, research on solar activity indicators (sunspot numbers, sunspot areas, flare index, filament and prominence, etc.) reveals that the magnetic activity in the southern and northern hemispheres shows amplitude asymmetry and phase in each solar cycle are out of synchronization. However, the hemispheric variations of the CMEs at different latitudes, in different cycles, and for different types (regular and specific events) are still unclear.

In this study, the researchers studied the hemispheric asymmetry of high-latitude and low-latitude CMEs in the time interval from 1996 January to 2020 December, based on the list of white-light CMEs in the Coordinated Data Analysis Web catalogue. The authors found that the regular CMEs are significantly correlated with solar activity expressed by international sunspot numbers, particularly for the low latitudes. However, the specific CMEs are not correlated with solar activity. The main reason for the hemispheric asymmetry of the CMEs is attributable to specific CMEs, not regular CMEs. The hemispheric asymmetry of high-latitude CMEs appears to have little connection to that of low-latitude CMEs.

The researchers also found that, for the total and specific CMEs, the relationship between the absolute asymmetry index at high and low latitudes has a positive correlation prior to the cycle maximum, but a negative correlation after the cycle maximum. Furthermore, the dominant hemisphere, the cumulative trend, and the amplitude of the total, specific and regular CMEs in cycle 23 differ from those in cycle 24. Their analysis results could be useful for understanding the cyclical variation of the magnetic free energy during different solar cycles, and could also provide insight into more physical processes responsible for the solar–terrestrial relationship.

Contact:
DENG Linhua
Yunnan Observatories, CAS
E-mail: lhdeng@ynao.ac.cn